Ammonia entry along the proximal tubule and loss from the loop of Henle appear to be critical to urinary ammonium excretion. The studies proposed will attempt to define the relative importance of the factors that influence ammonia entry and loss along the nephron. Potential factors influencing ammonia entry along the proximal or distal tubule include the tubule fluid flow rate, the rate of acidification, and the rate of ammonia synthesis. Previous studies have correlated entry along the proximal tubule with tubule fluid flow rate and this was enhanced by acidosis. However, since acidosis enhances all of the potential factors favoring ammonia entry, the relative importance of each of these is not clear. The factors favoring ammonia entry along the distal tubule are even less clear since there are conflicting reports on the role of flow rate in enhancing ammonia entry, the hydrogen secretory capacity of this segment is not well defined, and chronic metabolic acidosis has not been found to enhance entry along this segment. Similarly, the factors that potentially influence ammonia movement out of the loop of Henle including tubule fluid flow rate, tubule fluid pH, collecting duct acidification, and ammonium concentration, remain undefined. The proposed studies will then explore these factors using the technique of in vivo microperfusion. In particular, the ammonia permeabilities of the proximal and distal tubules will be defined as will the acidification rate of these segments in control and acidotic conditions. Ammonia entry will then be correlated with the rate of acidification, the tubule fluid flow rate, and rate of ammonia synthesis. Loss of ammonia from the loop of Henle will be correlated with tubule fluid flow rate, ammonia concentration, load of ammonia, and the presence or absence of chronic metabolic acidosis. The role of loop pH and collecting duct acidification in influencing ammonia loss will be determined by independently varying these factors. Thus, a comprehensive view of the factors governing ammonia movements along the nephron accessible to surface micropuncture will be attempted.